1. Field of the Invention
This invention relates to apparatus for establishing a uniform, predetermined charge on successive image frames of a charge-receiving element.
2. Description of the Prior Art
Although the corona-charging apparatus of the present invention has general applications, one preferred application is in the field of electrostatographic recording such as with electrophotographic apparatus (such as copiers or printers). In copier corona-charging apparatus, current from a corona-emitting electrode establishes a generally uniform electrostatic charge on an image segment of a charge-receiving element having a photoconductive insulating layer. The charged segment is then advanced to an exposure station where it is exposed to image-forming radiation to form a latent electrostatic image of a document to be copied. The latent image is thereafter developed and substantially transferred to paper upon which the copied image is to appear.
Various methods are known for adjusting the charge on a charge-receiving element by the corona-charging apparatus. For example, the speed of the charge-receiving element past the corona-emitting electrode can be varied to adjust its time of exposure to corona current. Another method for adjusting the charge is to vary the charging rate. Control electrodes such as grids, to which has been applied a potential approximately equal to that to which the charge-receiving element is to be charged are located between the corona-emitting electrode and the element. Current flow from the electrode to the element can be adjusted by varying the voltage of the grid.
Typically, the optimum grid voltage changes when reproducing successive original documents, to compensate for variances in background density of the original documents. Another application involves development of different image frames with different colored toners to be transferred to the same receiver sheet or to a different receiver sheet. Because of the difference in materials in the toners, they typically require different charging levels. Thus, adjustment of the charging rate between image segments is not new, and has been effected by changing the grid voltage after the trailing edge of one image segment has cleared the corona-charging apparatus and before the leading edge of the next image segment reaches the apparatus. However, this process undesirably requires the charge-receiving element to contain a physical transition region between the trailing edge of one image area segment and the leading edge of the next image area segment so that no portion of either image area segment is below the corona-charging apparatus during the period of grid voltage change. Thus, the image area segments on the charge-receiving element must be spaced apart by a distance equal to at least the width of the corona charging zone (i.e., the dimension of the charge-receiving apparatus in the direction of movement of the charge-receiving element).
The need for providing such spaces between the successive image area segments reduces the number of image area segments which can be located within a predetermined length of charge-receiving element. This in turn necessitates driving the charge-receiving element at a faster linear speed to make a predetermined number of copies per unit time than would be necessary if the image area segments were closer together in order to produce the same number of copies per unit time.
In order to overcome this problem, the prior art as exemplified by U.S. Pat. No. 4,695,723 suggests that a charging apparatus include electrodes for creating a corona discharge and a composite grid between the electrodes and a moving charge-receiving element. The composite grid includes a plurality of conductive controlled grid sets with at least one grid wire per set. The grid wires extend in a direction transverse to the charge-receiving element, and the voltage on the wires of each grid set is adjustable independently of the voltages on the wires of the other grid sets. Regulator means are provided for synchronizing the voltage of the wires of each grid set with the movement of the charge-receiving element. Thus, voltage on a grid set can be changed when that grid set is over the transition region between the trailing edge of one image area segment to the leading edge of the successive image area segment, and the transition region need only be as wide as the width of one grid wire set. While this greatly reduces the length of the transition region and increases the number of image area segments which can be located on a predetermined length of charge-receiving element, it does so at the cost of plural power supplies.
In U.S. Pat. No. 3,527,941, there is disclosed an electrostatic charging device which requires only one charging station but which applies a uniform charge at a relatively high rate of speed. The device comprises a corona discharger which is separated from a photoconductor by a grid. The grid produces a nonuniform field so that as the photoconductor is moved past the charging station it is subjected to a progressively lower potential which finally becomes the potential to which the photoconductor is to be charged. In specific embodiments disclosed in this patent, the electrostatic field produced by the grid is made nonuniform by (1) applying graduated voltages to the individual elements of the grid, (2) or by graduating the spacing between the grid elements themselves, or (3) by a combination of the above two methods. A problem with this device is that means must be provided for simultaneously applying different grid voltages to different grid elements and the assembly is required to be relatively large.
In U.S. Pat. No. 4,245,272, primary charging of a moving electrographic photoconductor to a nominal potential level is achieved with low sensitivity to variation in system parameters such as photoconductor capacitance, photoconductor velocity and/or charger efficiency. Separately addressed, AC corona discharge units are arranged and predeterminedly biased to first substantially overcharge the photoconductor relative to the nominal potential and then discharge the photoconductor to exit at the nominal potential level. As noted in this patent, at least two corona wires are required for first predeterminedly overcharging above the nominal voltage and second predeterminely discharging so that the photoconductor exits the charging station at the nominal level. Thus, this charging device of the prior art also requires separate power supplies.
In U.S. Pat. No. 2,890,343, an area charging device is disclosed for charging a xerographic drum wherein a charging unit is selectively turned on and off to provide a sensitized area with sharp leading and trailing edges of the voltage profile of the area. As disclosed in this patent, grid wires are electrically isolated from each other so that they can be subject to different bias potentials. A commutator is provided and synchronized with movement of the drum to sequentially change the potential on each grid during the commencement of charging of an area. Initially, all grid wires are at one potential (-800 volts) to block flow of charge to the drum. Thereafter, the potential is changed to +800) volts and that portion of the drum then passing beneath the grid wire is exposed to the corona discharge. Each grid wire is energized successively while the grid wires ahead of the energized grid wires are maintained at -800 volts, until the entire charging unit is turned on. At the trailing edge, the image frame process is worked in reverse. A problem with this apparatus is the requirement to separately electrically isolate each grid wire and the apparatus suggests a relatively large size charging unit in the direction of movement of the area being charged.
In U.S. Pat. No. 3,688,107 there is disclosed two embodiments of charging apparatus for rapidly and uniformly applying electrostatic charge onto an electrostatographic plate. In one embodiment, a stationary plate is located beneath a relatively large area charging scorotron. A screen wire grid is firstly biased to a very high voltage for a transient period defined by an RC time constant and thereafter the screen returns to its conventional bias level to bias the plate to that latter bias level. A problem with this apparatus is that it requires a very large charging device to charge an entire image frame area and it is inherently slow in terms of productivity because the plate is stationary during charging. In a second embodiment described in U.S. Pat. No. 3,688,107, a charging apparatus for charging a moving electrostatographic plate is described. In this second embodiment, a multiplicity of voltages is impressed upon the screen wires with the first wire having a large initial potential and other wires downstream having progressively lower potentials. It is thus apparent that this solution, too, requires separate electrical isolation of the grid sets.
In still another known approach a relatively high level of charge is provided by a corona charger and an auxiliary electroluminescent light panel is used to knock down the voltage to the desired V.sub.o.